The objective remains to elucidate high resolution crystal structures of electron transport proteins and enzymes which contain Fe-S clusters, heme groups and flavins. Methods of protein crystallography will be used. The results are relevant to studies of electron transfer nitrogen fixation and aerobic metabolism. Primary interests are the structure and function of 3Fe clusters, the protein environmental factors which determine the properties of Fe-S clusters, and structural aspects of electron transfer reactions in proteins. The structure of the Fe-S enzyme aconitase in the [3Fe-4S] state will be solved at 2.7 A resolution. The primary sequence will be determined through collaboration. Data will be collected on the activated [4Fe-4S] state of the aconitase crystals. The structure will be interpreted in relation to existing data on the enzyme mechanism and chemical states of the Fe-S cluster. The structure of Azotobacter ferredoxin will be refined beyond 2.0 A. The structure of the oxidized protein will be refined against data sets collected at pH 8 and pH 5. Data will be collected on reduced crystals as a function of pH. The structures will be analyzed in relation to data on Fe-S cluster interconversions and conformational changes in other ferredoxins and enzymes. Data will be collected to 1.7 A on oxidized and reduced Azotobacter cytochrome c5 crystals, and the structures will be refined. The structures will be compared to the refined structures of oxidized and reduced cytochrome c551.